A typical fuel tank for use in a motor vehicle for supplying fuel to the internal combustion engine thereof includes means for conveying fuel from the tank to the engine, and a filling connection usually in the form of a pipe which extends from the tank to a filling opening for refuelling of the tank. The tank further includes means for venting thereof during refuelling and during operation of the tank, which means can be referred to hereinafter for brevity as means for refuelling and operational venting. The tank further includes a fuel vapor filter by way of which venting of the fuel tank is effected.
In such a fuel tank the fuel vapor filter by way of which refuelling and operational venting is effected is usually in the form of an activated carbon filter. The hydrocarbon-charged gases from the fuel tank are condensed in the fuel vapor filter, hydrocarbons are retained in the activated carbon contained in the filter and the air cleaned in that way is discharged to the ambient atmosphere. When the fuel vapor filter is saturated with hydrocarbons, purging thereof is effected by the engine induction air being passed by way of the fuel vapor filter.
Basically it is desirable for the fuel vapor filter to be designed in such a way that as far as possible no hydrocarbons reach the atmosphere. In spite of the fuel vapor filter operating well however it is not possible to completely avoid that. The impurities which pass into the atmosphere through the fuel vapor filter are also referred to in this technical field as ‘bleed emissions’.
Therefore inter alia certain limits are set on the level of efficiency of the fuel vapor filter for the reason that, in a tank refuelling operation, depending on the respective filling speed, up to 60 l/minute of gas has to be displaced out of the fuel tank. That volume flow is passed by way of the fuel vapor filter. The higher the level of efficiency of the filter, the correspondingly higher is also the through-flow resistance thereof, so that the fuel vapor filter must afford a certain minimum flow transfer capability for the fuel vapors to flow therethrough in order to prevent the refuelling gun from prematurely switching off in the refuelling procedure.
In order to keep the flow through the fuel vapor filter at a comparatively low level and thus also to maintain the vapor loading thereof within certain limits in the refuelling procedure, it is known for a certain proportion of the gas volume flow which is displaced from the tank in the refuelling operation to be recirculated through the filling pipe, more especially through a conduit specifically provided for that purpose and referred to as a recirculation conduit. It will be noted however that such an arrangement does not make it possible to design the fuel vapor filter without having regard to the high gas volume flow which is to be displaced in the refuelling operation, in order to reduce the bleed emissions of the fuel vapor filter, especially as the structural volume of the filter is also subject to certain limits.